In metal halide lamps comprising a ceramic discharge tube held within an outer tube there is less reactivity between the discharge tube material and enclosed metals compared to quartz discharge tubes, which were in general use before the ascent of ceramic discharge tubes. Therefore, it is expected that a stable lifetime can be obtained for metal halide lamps comprising a ceramic discharge tube.
In the prior art, metal halide lamps having a discharge tube at both end portions of a transparent alumina tube that are closed by insulating ceramic caps or conducting caps are known as such metal halide lamps (see Publication of Unexamined Japanese Patent Publication (Tokkai) No. Sho 62-283543).
Further known are metal halide lamps having a ceramic discharge tube having end portions at both ends of a central portion and having a smaller diameter than the central portion (see Publication of Unexamined Japanese Patent Publication (Tokkai) No. Hei 6-196131). Electrically conductive lead-wires having an electrode at their ends are inserted at both end portions. The gaps between the edge portions of the discharge tube and the conductive lead-wire are sealed with a sealing material
Such conventional metal halide lamps using ceramic discharge tubes utilize the high thermal resistance of the ceramic to raise the tube-wall load (lamp power per surface area of the entire discharge tube) compared to metal halide lamps having a quartz discharge tube. It is known that by maintaining a vacuum inside the outer tube, the discharge tube temperature can be raised and the lamp efficiency can be increased. However, there has been no detailed research about the lamp efficiency and lifetime and their relation to the volume of the transparent ceramic constituting the discharge tube.
Because the volume of the transparent ceramic constituting the discharge tube in conventional metal halide lamps having a ceramic discharge tube is large, the proportion of the discharge energy that is thermally lost in the discharge tube is large, so that a considerable increase of the lamp efficiency cannot be achieved.
On the other hand, when the volume of the transparent ceramic constituting the discharge tube is made small to increase the lamp efficiency, the bond strength when the discharge tube is sintered into one piece becomes weak, so that cracks occur during the lamp operation, which lead to leaks in the discharge tube.
Moreover, to realize high efficiency and high color rendition, it is necessary to increase the discharge tube temperature and raise the metal vapor pressure inside the discharge tube. However, when the volume of the transparent ceramic material constituting the discharge tube is too small and a vacuum is maintained inside the outer tube, the discharge tube may be damaged due to heat-cycles during the lamp lifetime, because the discharge tube temperature is too high.